Whatever the nature of the mechanism by which the chuck is rotated, there is always a need for the workpiece (such as a drill or screwdriver bit) presented to the chuck to be held in a non-rotatable relationship thereto. One of the most frustrating factors in using a rotating workpiece held by chuck is the working loose of the grip of the chuck on the workpiece. Not only does this result in frictional wear between the gripping member of the chuck and the workpiece (which can result in extremely high temperatures and even cause melting of plastics componentry) but can often result in the workpiece becoming jammed in the item into which it is being drilled or screwed.
Both of the above problems have been addressed in the art, but with varying degrees of success. For example, in EP-A-0710520 there is disclosed a chuck for use with a powered drill or screw driver having locking members utilised to be moved between an engaged and a released position by rotation of the outer sleeve relative to a locking ring. In addition an engagement mechanism is disclosed having two engagement positions and is fitted between the sleeve and the ring. In one position the locking element is engaged, and in the other position it is released. There is also disclosed an intermediate ring having side parts and a collar to prevent any turning.
Whilst, in theory, the above mechanism functions to allow locking of the chuck once the jaws thereof are sufficiently tightened around a workpiece, in practice the componentry is highly complex and, therefore, both difficult and expensive to assemble. Furthermore, should any of the componentry malfunction, then easy and quick repair to the chuck is highly difficult.
An alternative method by which the chuck jaws may be locked tight to a workpiece in advance of use of the chuck is disclosed in EP-A-0519412. The chuck disclosed therein a plurality of angularly displaced detents are formed in either or both of the rotatable nut or the body assembly. The aim is for the balls of the bearing race to “drop” into the detents as the chuck jaws are tightened around a workpiece. In this way, apparently, the propensity for the chuck to become loosened during use is obviated.
Although the chuck disclosed in EP-A-0519412 is far more simple than that of EP-A-0710520, it still suffers significant shortcomings. For example, the choice of material for the nut or the body assembly (within which the displaced detents are formed) is critical. If the material chosen is too soft, then the balls can simply gouge out their own annular channel over a period of time during each tightening process, thereby removing the ability of the chuck to “lock”. Alternatively, if the material chosen is too hard, then it becomes brittle and can crack under the torque applied during the tightening process.
It is an object of the present invention, therefore, to at least alleviate the above-mentioned shortcomings by providing a more simple and effective means of locking the jaws of the chuck in the position in which they are securely fastened around a workpiece, as compared to the prior art. The need for an excessively complex arrangement and, therefore, the need for a large number of components to achieve this is avoided. Equally, the problems associated of the choice of hardness of the chuck materials becomes less relevant than in the prior art above.